Ceramic turbines are of interest for high-efficiency gas turbine engines because ceramic materials can tolerate higher temperatures than metals, leading directly to higher fuel efficiency. However, despite extensive research, ceramic turbines are not yet used in production engines due to several problems.
One major problem with ceramic turbines is structural reliability. Due to the lower fracture toughness (brittleness) of ceramic materials, small internal flaws or cracks have a greater tendency to grow over time when the material is under stress, eventually leading to failure. The larger the initial flaw or crack, the greater the propagation rate and the sooner the part will fail. However, if the initial flaw is smaller than a certain size, the “critical flaw size,” it will not grow at all, and the part will remain strong. Physically small turbine rotors have an advantage in this regard, as small flaws are easier to detect in small parts because they are larger relative to the overall size of the part. Additionally, if flaws follow a probabilistic distribution, as is typical for ceramics, then the probability of a greater-than-critical-size flaw occurring in a small part is lower, so the smaller turbine rotor is more likely to be flaw-free initially. Therefore, to those skilled in the art, small turbines are considered much more compatible with ceramic materials than large engines.
However, physically small turbine rotors also have their limitations. Small turbine rotors, particularly those with ceramics, high turbine inlet temperatures, and brazed joints, typically suffer from bearing overheating. In a large engine, the bearings are positioned at a relatively long distance away from the turbine rotor—many inches or even several feet—so it is easier to keep them cool. In contrast, small gas turbines, e.g., those producing under 30 kilowatts of shaft or electric power, typically have complete main shaft assemblies less than ten inches long. Therefore, at least one of the two main bearings must unavoidably be positioned inches from the hot turbine rotor, which makes bearing cooling a very difficult engineering problem. The higher the turbine inlet temperature, the more difficult the cooling problem. Ceramic turbines are only used when a very high turbine inlet temperature is desired, so invariably, bearing cooling is an extraordinarily difficult design challenge in these cases.
In large engines, the bearing overheating problem is typically solved by having oil supply systems and oil coolers. Therefore, in these large engines, it is generally convenient to use the oil to keep the bearings cool, as well as to lubricate them. However, in small, simple engines, this is approach is undesirable because it makes the engine more complex; and thus, more expensive and more prone to failure.
Despite the complexities of the oil systems, almost all small engines, like large engines, use the lubrication system to keep the bearings cool. Typically, they utilize a system to spray a mixture of fuel and oil on the bearings in a “total-loss” lubrication system. To simplify the system, they typically will mix all of the fuel with oil, so that a single fuel/oil pump can supply the bearings and the main combustor with fuel/oil mixture. However, this system tends to lead to additional problems, such as carbon formation, smoke generation in the combustor, and fuel injector coking.
In total loss systems, after going through the bearing, the fuel/oil mix must flow into the combustor and burn. This requirement forces the bearings to be positioned near the combustor, exacerbating the thermal problems. Because of these constraints, small turbine engine bearings typically operate at steady-state temperatures around 300 degrees Celsius, which greatly reduces their load capacity and increases the wear rate. Most small engine bearings need to be replaced approximately every 25 hours. With higher operating temperatures and ceramic materials, this already short life would surely be reduced even further.
Another major problem with ceramic turbines has been the difficulty in joining ceramic turbine rotors to metal shafts. Ceramic materials that have good high-temperature strength and creep properties have a much lower coefficient of thermal expansion (CTE) than metals. For example, silicon nitride (Si3N4), known to those skilled in the art as presently the best turbine-grade ceramic, has a CTE of 3.1 μm/(m*K). However, a typical stainless steel shaft material has a CTE in the 11-17 μm/(m*K) range. Therefore, when the turbine and shaft get hot, the metal can grow 4-5 times as much as the mating ceramic part. If the ceramic turbine and metal shaft are bonded together rigidly, this can cause large stresses that can break the ceramic material or yield the metal, causing the joint to fail.
On the other hand, if the ceramic turbine and metal shaft are not bonded rigidly, they can move relative to each other. This configuration could be acceptable if the geometry of the joint ensures that the turbine rotor and shaft remain concentric and strongly connected, both during operation and after repeated start/stop cycles. However, cylindrical joints, which are typically the most common type of joint most likely due to its apparent simplicity, cannot maintain concentricity and strength when the two cylindrical parts repeatedly move relative to each other. Therefore, cylindrical joints that move during operation can quickly fail.
One solution to avoid this problem can be to replace the steel shaft with a ceramic, or at least a low-CTE metal, so that the two shafts could have nearly equal CTEs; and therefore, would most likely not generate large thermal stresses. However, the substitution of the ceramic shaft approach is rarely used for various reasons. For example, at room temperature, metals are stronger and tougher than ceramics. They are also far more easily machined to precise tolerances in complex shapes such as gears, splines, keyways, and other typical features of rotating shafts. Furthermore, other metal parts, such as bearings, must invariably be mounted on the shaft. These mounted parts also need to have a close CTE match with the shaft in order to maintain a tight fit that does not loosen as the shaft heats up during operation. For all these reasons, shafts made from steel, or other strong/tough/machinable metal with a CTE value near steel, are typically preferable to ceramic shafts; and therefore, are used almost universally.
In the prior art, ceramic turbines have been mated to metal shafts successfully in the past. The most common joining method is brazing. In this method, an “active filler metal” with a lower melting point than either base material is used to bond the ceramic to the metal substrate, similar to how solder is used to join electrical wires. The filler metal is heated beyond its melting point, flows into the space between parts, and when it solidifies, adheres, and bonds to both the metal and the ceramic. This process only works if the filler metal “wets” both materials, which is a constraint that severely limits the range of choices for the filler metal. To provide thermal strain tolerance, the filler metal should also have a CTE somewhere between the CTEs of the parts being bonded, which is another limiting constraint. Finally, the filler metal should be relatively soft, i.e., low Young's modulus, in order to provide a small amount of compliance to the joint.
Few filler metals are available that have all of these properties. The current state of this technology as provided at azom.com, states that, “The majority of the commercial active metal brazes have been developed for moderate temperature use up to −450° C. However, one of the many attractive properties of ceramics is their ability to survive high temperatures—for example, alumina typically has an upper use temperature of 1700° C. The braze alloys used therefore need to have higher temperature capability than is currently available. One method is to coat the ceramic with either a reactive or refractory metal (WMo, Ta, Cr) then braze using high temperature braze alloys, such as palladium and platinum-based systems. This . . . has been used successfully for joining many high temperature ceramics.
Therefore, according to the prior art, brazed joints must be designed carefully using finite element analysis (FEA) to analyze the stresses generated during thermal expansion. The geometry of the joint, the thickness of the filler metal layer, and the cleaning and fixturing of the bore and shaft during the brazing operation are all critical. Therefore, if not done properly, the process can result in joint failure.
Despite all these limitations, brazing remains the most common method of joining metal and ceramic shafts. However, the problem of keeping the bearings cool still exists, particularly for small engines as explained above. In a brazed shaft joint, the metal, the ceramic, and the filler are all typically good heat conductors. (It should be noted that while some ceramics are insulators, silicon nitride is not. Its thermal conductivity typically exceeds 20 W/m-K, which is on the same order as steel.) Furthermore, to ensure sufficient room for a high-strength bond, with a thick enough filler metal layer to allow for some strain tolerance, the size of the joint must be fairly large. Therefore, the result is that the cross sectional area for heat conduction is also large. Since shaft dynamics considerations limit the maximum length of the shaft overall, and in particular the maximum distance between the turbine rotor and the bearing, it is not possible to simply use a long shaft to insulate the bearing from this heat conducted from the turbine. It is also difficult to squeeze a thermally insulating feature into this very constrained space on the shaft.
A final problem with brazed joints is that they cannot be disassembled. Bearings on high-speed shafts must fit very tightly, to minimize play and ensure concentricity. Therefore, once assembled, the entire rotating assembly can be especially difficult to take back apart. This can make it very difficult to design a gas turbine engine that can readily be repaired easily and quickly.
In summary, brazed and adhesively bonded joints are permanent, and cannot be easily disassembled. They can be difficult to design manufacture, and they typically conduct too much heat to the bearings, which is unavoidable due to shaft dynamics considerations. This problem is particularly severe in small engines; and therefore, the bearings of ceramic turbine engines, particularly small ones, tend to fail often and need frequent replacement.
Accordingly, there remains a need for an alternative method of joining ceramic turbine rotors to metal shafts. Preferably, this new method would allow thermal strains to be accommodated, to limit stresses. It would ideally provide for quick and easy disassembly/reassembly. When assembled, the joint could assure very precise alignment and concentricity between all rotating components. However, the geometry of the joint should also accommodate thermal strains that inevitably arise due to different thermal expansion coefficients and heating/cooling rates of the mating components. Finally, the joining method should ideally provide a significant amount of thermal insulation, in order to allow the bearing near the turbine rotor to remain relatively cool, even while the engine is running.